The present invention relates to large arrays of electrostatic actuators that need large actuation voltages. More particularly the invention relates to a method and apparatus for controlling large arrays of over one thousand electrostatic actuators using actuation voltages of hundreds of volts, while providing fine control of the voltage of each actuator at minimum power and size.
With increasing MEMS devices being accepted in the market place, the diversity of required drive/sense electronic circuits has increased. One of the most challenging aspects of this product development is the need for fine control of the voltage of each actuator (out of a thousand or more) while using minimum power. At the present time, such a method and apparatus does not exist in the commercial market.
Gugler et al. U.S. Pat. No. 1,123,275 broadly disclosed the general concept of a combination of a voltage source, a regulating switch, and multiple loads. This reference teaches the need for having power being transmitted to multiple loads, but, of course, does not recognize the needs of electrostatic actuators of any size.
The use of a plurality of voltage ramps is disclosed in Mallinson et al. U.S. Pat. No. 5,321,404, for use with an analog to digital converter. It does not contemplate large numbers of voltage outputs through switches. Chu et al. U.S. Pat. No. 5,945,870 discloses high voltage generator circuits with well controlled ramp rates, particularly for use in electrically erasable programmable read only memories. It does not suggest charging and refreshing capacitors simultaneously at the voltage needed for the allocated actuators.
Also, Tailliet U.S. Pat. No. 6,157,243 discloses the generation of charges in a charge pump device with saw tooth shaped clock signals. Though the concept of generating a large voltage (in this case ten to twenty volts) is disclosed, the simultaneous use of that voltage, or voltages larger by a factor of ten, is not considered. Staircase high voltage is disclosed in Barkow U.S. Pat. No. 3,619,647. Seesink U.S. Pat. No. 5,483,434 teaches a series of diode and capacitor combinations are connected such that voltage is multiplied to produce a high voltage output at the last capacitor.
As can be seen, none of the prior art suggests large array electrostatic actuator control, and more particularly, none of the art suggests a system in which voltage drift can be compensated for when the electrostatic actuator, capacitor and electronics experiences leakage.
The use of very large arrays of electrostatic actuators is not a well developed field yet because it has been difficult to control all of the arrays in a period of time short enough to accomplish the objectives of the device using the arrays. For example, a large array may be usable in a polychromatic device where 1024 beams are provided, each with an electrostatic actuator and each with a mirror, such that pure white light can be reflected from the mirrors to generate a particular spectrum pattern. By comparing known spectra to observed spectra, the user could identify which particular material, such as a gas remote from the observer, is being deployed. Another potential for large arrays, in the 1000s, would be large antennae that might be launched into space folded like an umbrella for launch, then expanded and positioned when in orbit. With proper control of the electrostatic actuators, an antenna of diameter of one hundred meters or more could be deployed. Proper control of this multiplicity of actuators would be necessary for these such devices to properly function as intended.
It would be of great advantage in the art if a method and apparatus for controlling large arrays of electrostatic actuators could be provided.
It would be another advance in the art if the method and apparatus would include the use of a large number of capacitors, one for each electrostatic actuator of an array, such that the capacitors would be charged simultaneously at the voltage needed for the allocated actuator.
Yet another advance would be to have a method and apparatus for responding to the voltage leaks that actuators and capacitors experience over time, such that even arrays of tens of thousands of actuators cold be controlled.
Other advantages will appear hereinafter.
It has now been discovered that the above and other objects of the present invention may be accomplished in the following manner. Specifically, the present invention provides for at least one high voltage generator for a set or array of n electrostatic actuators, where n is an integer of significant size, such as at least 32 or 100 or 1,000 or even 10,000. Each of the n electrostatic actuators may optionally include a capacitor connected in parallel to maintain the actuator voltage. Each part of the array is to be initially charged to a desired operating voltage and then periodically recharged to that desired voltage to replace voltage losses due to leakage in the actuator. A low leakage capacitor can help the actuator keeping the ripple of the voltage between two refreshed cycles under the desired value.
Through experimentation or calculation, the specific voltage leakage for each element in an array is determined. The high voltage generator is connected to the parallel connection of the capacitor and the electrostatic actuator through a switch. The switch is controlled by a central processing unit, which can be any computer programmed to issue commands. Other than during the initial charging step, the switch operates during the time when the known leakage voltage is to be added to the line. The state of each switch is controlled by the information of the latch register. The load of the latch register occurs after the information is serially loaded into the shift register. Other than during the initial charging step, the switch operates during the time when the drop voltage generated by the leakage current is compensated in the refresh cycle.
The high voltage generator provides a voltage source that is determined by the needs of the device employing the array. The shape of the voltage and the time of the voltage rise is defined by the end use and the application of the array. The central processing unit is programmed to set the amount and form of the voltage, as well as to control the time for the switch to be open and closed.
When the design employs a relatively large number of actuators, such as in the thousands, one may decide to use more than one high voltage generator, each for n/2 or n/3 actuators for example. The advantage is that the charging time of the actuators will be twice or three times as fast and the disadvantage is the higher consumption of power and the bigger size of the circuit. This will reduce the total actuation time. The extreme of this concept is to have n high voltage generators, which may work for small numbers but would be impossible for thousands or even hundreds of actuators.
When the design employs a relatively large number of actuators, such as in the thousands, one may decide to use more than one Serial Data line generated by the CPU, each for n/2 or n/3 cells of the Shift Register. This means the Shift Register is split in two or three smaller shift registers that are simultaneously serial loaded. The advantage is that the loading time of the data in the shift register will be twice or three times as fast and the disadvantage is the higher power consumption and size of the circuit. This will reduce the total actuation time. At the limit the initial register can be split in n registers that it means the information will be parallel loaded instead of serial which may work for small numbers but would be impossible for thousands or even hundreds of actuators.